Carbon Flow in Acidic CO₂ Electroreduction

Electrochemical CO₂ reduction in acidic media attracts extensive research attention due to its potential in increasing carbon efficiency. In most reports, alkali cations are introduced to suppress hydrogen evolution and to promote CO₂ reduction. However, the mass transport of alkali cations through cation exchange membrane induces the change of electrolyte compositions. Herein, the variation of electrolyte compositions and the flow of carbon during CO₂ reduction are analyzed quantitatively by simulation and experiments. If the initial amount of alkali cations in the anolyte is higher than the initial amount of H⁺ in the catholyte, the pH of the catholyte increases remarkably in long-term CO₂ reduction electrolysis, resulting in the decrease of carbon efficiency. Bicarbonate salt precipitation on the cathode with alkali cation-containing catholyte is another origin of the decrease of CO₂ reduction Faradaic efficiency and carbon efficiency. To maintain high carbon efficiency, the electrolyte should contain low concentration of alkali cations or even be free of alkali cations. Decorating the catalyst of cathode with ionomer with high density of cation sites enables CO₂ reduction in pure acid solution, achieving 30-h stable carbon efficiency.